1. Field of the Invention
The invention relates to catheters and more particularly, the invention relates to a microcatheter having a varying flexibility along its length.
2. State of the Related Art
Numerous conditions in the vasculature may be treated or diagnosed with intravascular catheters. Aneurysms, arterio-venous malformations, fistulas and occlusions may be diagnosed with angiography, in which a radiopaque dye is injected into a blood vessel through a catheter while the blood vessel is imaged under fluoroscopy. Stenosis and other occlusive diseases can be diagnosed with ultrasound catheters inserted into the blood vessels. Some vascular maladies, such as occlusions, lesions, tumors, or diseases can be treated with injection of therapeutic agents in the blood vessel through a catheter. Some aneurysms and fistulas may be treated with the injection of embolic materials into the blood vessel. Each of these techniques, and various other techniques, require the insertion of a catheter into the blood vessel. The catheter is preferably inserted into a large remote blood vessel such as the femoral artery and navigated through the vasculature to the target blood vessel.
When the catheters are intended to be inserted into the very small blood vessels of the brain, heart, kidneys, and the like, which are far away from the insertion point and require passage of the catheter through the many twists and turns of the vasculature, guide catheters and/or guide wires are used to help insert the catheter into the target vessel. Typically, a guide wire and catheter are inserted into the femoral artery using the Seldinger technique (or a cutdown). The guidewire can be pushed and twisted to navigate the catheter through the femoral artery, up the descending aorta toward a target site. When accessing the vasculature of the brain, the guidewire and catheter are steered into the aortic arch, then into one of the carotid arteries or vertebral arteries, and then into the brain. Once a tip of the catheter has been placed at the target site, either within the diseased blood vessel or nearby, the guidewire is removed for injection of a therapeutic agent.
The process of tracking becomes more difficult as the guidewire, catheter, and target vessel become very small, and when the target site is deep within the vasculature and involves a highly tortuous path for access. Most small blood vessels of clinical significance must be accessed though highly tortuous pathways. For example, arching over the aortic arch and turning into a coronary artery to reach the distal portion of the coronary arteries, or turning into the carotid arteries or the vertebral arteries, into the Circle of Willis and into the cerebral arteries are common vasculature catheterizations though highly tortuous pathways. To reach a target vessel, the catheter must be quite flexible in order to follow the tortuous path and at the same time, the catheter must be stiff enough to allow the distal end of the catheter to be manipulated from an external access site, which may be as much as one meter or more from the targeted vessels.
In the case of pathological tortuosity, such as when the deep vasculature involves turns in excess of 90.degree. when branching off from one blood vessel to another blood vessel (paths which branch off the preceding vessel at angles greater than a right angle), and where the total path length within the target tissue is at least about 5 cm and is also characterized as being accessible by a guidewire 18 mil or smaller of the type described above, but being too delicate and/or tortuous for accessing by a particularly larger diameter guide wire, the problem is particularly acute.
Placement of catheters in positions deep within the vasculature is quite difficult without the aid of a guide wire, and tracking over a pre-positioned guidewire is often hampered by kinking and columnar collapse of the catheter on the guidewire.
Previous attempts to solve this problem include Engelson, Catheter for Guide-Wire Tracking, U.S. Pat. No. 4,739,768 (Apr. 26, 1988). Engelson proposes the use of a two or three segment catheter where the proximal segment is stiff and the distal segment, which must be at least about 5 cm long, is flexible, and in fact is so flexible that it is incapable of being inserted into a blood vessel without the aid of a guidewire. The catheter is constructed with at least two layers, with the outer layer being more flexible that the inner layer, and extending at least 5 cm past the distal end of the inner layer to form a flexible distal tip should have a gradually increasing stiffness from the distal to the proximal end of the distal segment.
Other methods of controlling the stiffness of different regions of catheter tubes have been proposed. Bos, Catheters with Variable Properties, U.S. Pat. No. 5,456,674 (Oct. 10, 1995) shows a catheter with one or more longitudinal bands of material running the length of the catheter. The bands may be made with differing flexibilities, and the length of each band may be controlled through the operation of cutoff valves during the extrusion manufacturing process. For example, in FIG. 5, Bos illustrates the construction of a multiple segment catheter, where each segment has a band of stiff material, and an intermediate segment of intermediate stiffness has an additional band of stiff material, and the third, stiffest segment has yet two more bands of stiff material.